Electron multiplier



- P 1944- s. VAN MIERLO 2,346,952

ELECTRON MULTIPLIER- Filed Nov. 18. 1936 3 Sheets-Sheet 1 IHVEHIOK I 53 mm M/elzo manner April 18, 1944. s. VAN-MIERLQ 2,346,952

ELECTRON MULTIPLIER Filed Nov. 18. 1936 a Sheets-Sheet 2 Fig.7 v

3 23 'ivmMmzo ATTORREY April 1944. s. VAN MIERLO 2,346,952

ELECTRON MULTIPLIER Filed NOV. 18, 1936 5 Sheets-Sheet 3 Fig. I I3. s gug z w mromx Patented Apr. 18, 1944 ELECTRON MULTIPLIER Stanislas Van Mierlo, Paris, France, assignor to International Standard Electric Corporation,

New York, N. Y.

Application November 18, 1936, Serial No. 111,429 7 In France December 28, 1935 21 Claims.

The present invention relates to electron discharge apparatus and particularly to such apparatus involving secondary electron emission.

The invention makes use of the" secondary emission of electrons produced by the bombardment of a body by primary electrons or under theinfiuence of radiations such as light radiations or in any other manner. All substances, even the insulating materials, can emit these electrons under certain conditions.

In order that the number of secondary electrons may be as high as possible it hasbeen found desirable to fulfill the following conditions:

(1) To use as the emissive surface an unpolished surface preferably composed of electropositive metal such as caesium or by a basic metal covered with a film of electro-positive metal (2) To employ speeds of primary electrons of the order of 400-600 volts.

Under these conditions one can obtain an emission of about secondary electrons per primary electron.

It is further known that these secondary electrons have a rather low initial speed (corresponding to a tension of several volts) and that they are emitted in all directions. The temperature has very little influence upon the emission.

It is clear that the phenomenon of secondary emission can be utilised for the amplification of an electric current and several proposals have been made in this connection. In the case in which it is desired to obtain very high amplifications, tubes with several electrodes can be constructed and arranged in such manner as to produce several successive multiplications.

This may be done in two ways:

1. A number of electrodes are arranged in accordance with a closed arrangement (for example, a circle). In such arrangements the electrons are displaced from one electrode to the next and from the last electrode again towards th first.

The simplest case is that of two electrodes disposed opposite to each other, the electrons having a tc-and-fro movement between the electrodes.

2. A number of electrodes are placed in accordance with an open arrangement (for example a straight line), the electrons being displaced from one electrode to the next and when the last is reached passing into an output circuit.

The first method has the advantage of employing fewer electrodes because certain or all of them are utilised several times. It has however the disadvantage that it is more difiicult to cause the electrons to pass into the output circuit in such manner as to obtain a stable condition of functioning. In both cases it is necessary to arrange the circuits so that at certain moments a given electrode may be more positive than the preceding electrode which is emitting the electrons.

When the electrodes are placed in accordance with the closed arrangement, it is necessary cone. tinuously tochange the potential of theelectrodes in order that the electrons may find a. higher potential when theybegin anew. In the case of.

two electrodes, they can, for example, be connected .with an oscillating circuit. arranged so thateach time the electronsare emitted by one of the electrodes, they find the other electrode more positive. The frequency of this oscillating circuit will-be chosen taking into consideration the potential and the distance-between the electrodes.

When the electrodes are placed in accordance withan open arrangement, two cases may be considered:

1. The potentials of the electrodes have a fixed value: the potentialis more positive forth second; electrode than for the first," for the third than for-the second, etc.

2. The potentials of the electrodes vary so that there is, so to speak; a wave ofpotential being displaced from the first to the last electrode, and attracting the electrons behind it.

Other cases may be considered when one utilises intermediate grids, as will be explained later.

The advantages of the second method is that it is possible to drive all the electrons emitted by one electrode towards the next, while this does not seem possible with the first method.

The invention will be better understood by means of the following description given in relation with the accompanying drawings in which:

Fig. 1 shows schematically an electron multiplier tube having several electrodes, certain of which are fed with alternating current;

Fig. 2 represents a diagram of the distribution of the potentials on the electrodes of the tube of Fig. 1;

Fig. 3 represents a diagram showing in various cases the variationv of thespeed of the electrons by function of wX;

Fig. 4 shows the variation under difierent conditions of the quality of wX by function of wt;

Fig. 5, represents the variation under different conditions of the speed of the electrons by function of wt;

Fig. 6 represents a diagram permitting the best operative conditions of the system to beobtained;

Fig. 7 represents curvesshowing the distortion due to the-fact that the electrons are not. all animated with the same speed;

Figs. 8 and 8A represent in elevation. and in plane a construction of an electron multiplier tube;

Figs. 9 and 9A represent another arrangement of the electron multiplier tube;

Fig. represents schematically another form of construction of. an electron multiplier tube;

Figs. 11A and 11B represent difierent methods of connecting the electrodes of tubes employing the characteristics of the invention;

Fig. 12 shows a diagram of the distribution of the potentials for a given wave form on the electrodes of a tube employing features of the inven- 1 tion;

Fig. 13 represents a method of connection of two tubes employing features of the invention;

Fig. 14 represents a tube according to the features of the present invention, employed as radioelectric wave transmitter;

Fig. 15 shows a construction in which the dimensions of the various electrodes are different and in which a final grid is provided.

In Fig. 1 the cathode C emits electrons which are controlled by the grid 9. The sources S,-S, S",. S' of alternating current are displaced by 90 from eachother. The electrodes have thus successively a maximum potential with respect to earth. The electrons arriving on I produce secondary electrons, which, during one period of time are attracted towards 2, etc. Finally the electrons emitted by I arrive on the collecting electrode A, which communicates with the output circuit. In this case the latter electrode has been connected to a source of direct current B. It is to be understood that variable tension could also be employed.

As has already been remarkiedit is necessary that the potential of the electrodes which emit the electrons be smaller than the potential of the next electrode and greater or equal to that of the preceding electrode at least during a portion of the time taken by the electron in passing from one electrode to the other. As, at the moment of the arrival of the electrons on an electrode, the same conditions are applicable for this latter electrode, which then becomes an emission electrode. it must be taken into consideration that the variations of potential may be the same for all the electrodes, but they must be dephased. Asthe potential cannot increase indefinitely, it is necessary to make use of periodical variations and I shall consider herein below in a more detailed manner sinusoidal variations; although other types of periodic variations may be more. important-(for example successive rectangular impulses or saw-teeth impulses).

In order that an electrode may during a portion of the time be more positive than the preceding electrode and less positive than the following electrode, I may consider dephasings of 90 or of 120. For 180 the electrodes adjacent to those considered will be on the same potential, which is not generally desirable.

In order to examine the operation more closely we will suppose that the displacement is of 90. Fig. 2 shows in space and time how the potentials of the electrodes vary.

Above the drawings are shown electrodes vC, g, l 2, 3, 4 and A, imagining that they are spaced in a uniform manner. The vertical lines traced below the electrodes serve as time axis in order to represent the potential variations of the corresponding electrode. These variations are represented by sinusoidals displaced by 90. An electron emitted by C at a moment D will find the electrode l more positive and will follow a curved path DE. This electron should find at I a potential equal to the potential'which the electrode 0 had at the moment when the electron has quitted.

When this condition is approximately fulfilled the electrons will reach th other electrodes at the moments E. F, G. H. I. In the case in which the electrode C produces electrons all the time, they will only be attracted towards l during a certain period as will be seen later. The electron current will thus be modulated at a frequency corresponding to the frequency of the variations of potential. Fig. 2 represents three paths of electrons D I, D I, D" I", at successive periods.

There is obviously a certain relation between the distance of the electrodes, the frequency and the potentials.

When an electron without initial speed is placed in an electric field, E, its speed after a journey S is implicitly given by the equation:

g figg g g Approximate volts Speed (Tm/Sec.

In order to obtain the maximum of secondary electrons, the difference of potential must be of the order of. 400 to 600 volts, and the corresponding speed is then about l3 10 cm./sec.

It will be seen that the time taken by an electron in order to pass from one electrode to the next is very small and with a .distanceof some centimetres it is necessary to employ frequencies of several megacycles.

In the present case the conditions are more complicated because the electrodes are not large parallel plates (and the field is thus not uniform), because the difference of potential continually varies, and because electrons have an initial speed in any direction. It will, however, be sufficient for a first approximation to consider uniform fields and an initial speed of zero, the sole variant being the difierence of potential.

As we are imagining that the potentials of all the electrodes with respect to earth vary in accordance with a sinusoidal law, the difference of potential between the adjacent electrodes will also be sinusoidal. In the case in which the initial speed of the electrons is zero, they will begin to be attracted towards the next electrode at the moment when the latter becomes more positive. In the case in which this electrode remains more positive during the whole time of their path, their speed will increase continuously. In the case in which this electrode remains negative at a given moment, their speed will diminish. In certain cases the electrode can undergo such a delay that the secondary electrons emitted when they strike the next electrode can no longer be effectively employed. In other cases the electrons may not' rea'ch the: foliowin'girselectmde at: all? but return to the eleotrofi wlrilch fias efi'litted' them; For: pertain "relations betweemthelpotei e: tial, the; distance, and thettfrequenew the ciency' of the 'system 'wlll he theg'reaiteeti following calculation may assi'sflin determiningthese relations":

in which xis the. distance through worm the: electron-has traveled, \V is thesp'eed oftheelec- The curves in "Fl'gsl' 3i 4'"a:n'd 5" respeotively'reij resent thespeed as a fimction of the distance; the distance as a'functio'n' of 'the'time-and fhe sp'ee'cl'ase function of the time forthevahies" and for the intermediate-values "of" In order to obtainthe"maximumemoieney the electrons emitted at the moment ep:- =0"'must"not take a,- time less than in orderto reach the following eleotrode= =(othe1' wise the" secondary-electronsproduced' by-i the latter will not beuseful) and the'electrons emitted a-fter must not take more time than to reach thenext'e1ectro'd*-'(otherwlse'thesec onda'ry electrons will arrive too late);

In Fig. 6 the'vertic'ail lines-l, 2" and 3' r'e'p";

The distance -neoeseary between*the electrodes" mambo"obtained-by a g raph of Fig;- 4 one must take into conslderati on' the distance wXi' corresponding" to the c '--i'n= which electrons leaving: an electrode at- 0 re'ach the" el'bt'rlode Mar-ent r; thspebdmf the'" eleotrorls 'arriving on": one: of the eleetrodesmtist': be of' the'"o1fder' or: 13 x10" cm/see." as exolaixred' herein above."

From Fig. 3 it ls-"clear that at a'=dista"nc'e the-spee oiil the average equal to A.-

repeated at various times, the concentration will increase more and more. x

In this simplified case (uniform fieldzero,

pulses will become shorter as indicated by the 1 curve c. It is, of course, understood that for non-uniform fields and for initial speeds which are not zero, the conditions will be changed; thus for example certain electrons leaving I before =0* may reach 2, a nd the difierence be; tween the time which the different electrons take; multiplier electrodes, by changing the potential to make the journey will be greater than in the present case.

In the foregoing, space charges have not been taken into consideration whichmore or lessmodify the conditions of operation.

The multiplier-tubes may be constructed in various ways. Three examples .-of relati-vear rangement of the electrodes ar given in the following.

A series of parallel electrodes may be employed as shown in Fig. 8. Four glass rods 5 support a series of parallel electrodes l, 2, 3,4, l"=,-2,- 3343A, which may be constructed like the grids .in'vacuum tubes. The conductors 6 bring the alternating current to the electrodes- The first electrode Ph may consist of a mica plate covered by a transparent layer of silver and caesium and treated so as to form a photo-sensitive surface.

The electrode unit is fixed by a ring 1 on the stem of the tube.

special form' and'ra superposed direct :tension. The difference'ofpotential must not be the same between-the successive electrodes.- -In.-the same way the distances may .vary. For a given tubethe tension will-,be regulated so as to obtain the:

maximum output.

At certain places theelectrons may be divided I parallel and in this case again the-phases may or may not coincide.

In the case of a dephasing of 180 an arrangement is obtained which will recall the well-known push-pull connection.

The amplification of these tubes may be varied during operation by changing the number of of the grids, or by changing the tension of the electrodes.

In the case of an open arrangement a much greater stability of operation may be obtained by means of variable potentials than with fixed po-. tentials. That "is, wit'h"fixed potentials'a portion of the secondary electrons emitted by a given electrode cannot be attracted towards the next electrode owing to the existence of an electrostatic field between the electrode which is emitting and the preceding one. Certain electrons may thus be forced to return towards theelec.

In this arrangement the majority of the second- I ary electrons are emitted by the upper surface of the electrode, while they must be displaced downwardly. In order to avoidthis disadvantage the-electrodes may be formed by circular wires or by tubesof any-section and of short lengths arranged in parallel. 1

A series of plane electrodes or slightly concave or convex electrodes also could be employed arranged in a circle or a helix. An example of this latter arrangement is shown in Fig. 9. The electrons emitted by a cathode C and =governed by the grid g are directed towards electrodes l, 2, 3, 4, l, 2',-3', 4', A. These various electrodes are supported by a series of mica discs D.

As the electrodes land I, 2 and 2, 3 and 3, 4 and 4 are connected together two by two, one may also employ fairly long plate as shown in Fig. 10; the electrons will follow a helicoidal path under the influence of a central magnet M, and each electrode will be employed, twice by the electrons before reaching the collecting electrode A. It will be taken into consideration that this method of proceeding more or less resembles -a closed arrangement, except that a definite output has been provided for the electrons; This arrangement may be considered as a combination ofaclosed and open arrangement.

In these tubes plates may be employed which reflect the electrons, or screens and electrodes which modify the electrical field so as better to direct the electrons towards the next electrode. Magnetic fields may also be employedfonthis purpose. I

For the polarisation of the electrodes ordinary alternating current or rectified alternating.,.cur-

rent may be employed or a periodic'cui rent of.

trode which has emitted them. Slight variations be cancelled completely. In order to obtain this condition a single positive impulse is successively given to the various electrodes until the moment when the electrons arrive on the collecting electrode, ,Only at this moment is the cycle of operationsbegun again. The electrons emitted by the first electrode would only be employed for a fraction of the time, but one would have the advantage of employing at each multiplication all the secondary electrons, and the consequent advantage that the gain per stage and the stability will be'very great. In order to give a clear picture wewill consider here 7 multiplications of 10 and we will suppose that only a tenth of the electrons emitted by the first electrode are employed. The total amplification will thus be 10 or db. In order to obtain the same amplification with a tube in which there are only 5 useful' electrons out of 10, at'ea'ch multiplication 9 multiplications will be necessary. If the number of electrons varies from 5 to 4, the amplification will vary by about 20 db. It will thus be seen that more stability is obtained with fewer stages in the first case. v

The spaced impulses may be produced in various known ways, for example, by means of a tube whose grid has a. negative high tension so as only to pass the peaks of the alternating current. The method of variable potential can also be combined with that of constant potentialQ For example, by using multiplication electrodes of fixed potential and intercalary electrodes or grids intended to cause the field to varyin the direction indicated above.

Thus during the period in which the electrons touch one of the multiplication electrodes, the preceding grid may be of the same potential (or approximately), while normally this grid ismor'e negative.

Figs. 11A and 11B give two examples of a tube with grids of variable potentials and multiplica cordance with this method one may consider the tube as consisting of a series of elementary tubes which operate in turn and are all connected in cascade without external coupling elements. The electrodes operate sometimes as cathodes, sometimes as anodes. The separations between the elementary tubes are efiected by means of grids.

- If electrons are continuously emitted by thefirst electrode they will be stopped in part by the grid or grids during certain periods.

This device may .be improved by inserting several grids. In the case of two grids, the first may have as main function to give an initial speed. to the electrons and thesecond may have as-its function to prevent a field reaching the next electrode, at least during the period when thiselectrodeemits secondary electrons.

The grids may be given variable potentials'pe-' riodically, either in accordance with a sinusoidal law (with or without constant polarisation) or accordance with another more complicated law.

The distancebetweena grid and the next multiplying electrode couldbe such that the time taken. by an electron to pass through this-space isof the order ofa half-period ofthe-variations of-potentialof thegrid. The-latter will (in the case-of rectangular variations) assume the potential of the multiplication electrode at th'emo ment when'the electrons arrive on this electrode and'will return to the negative potential atv the mb'mentwh'en the last electrons will haverea'ched the "multiplication" electrode, as is'showri in Fig. I2, whereEi and E2 are two successive multiplication electrodes, Gisa grid, A A the pathfof thefirstelectron from a" given group and BB th'e'p'ath of the lastelectrono'f this group; The grid is'at the potential of E2 during the period T; In the case of several grids we can see";still better the analogy with a series of elementary tubes connected in" series. The whole or the present art of multipleelectrode tubes may be applied in the present case.

In the case of power tubes, the latter maybe connected to a first multiplier'tube. One of" the first problems which arises is thus that of'the connection of these two tubes with each other. Thetwo tubes need variations of potential-ofthe same frequencyandi the phases must thus have a certain relation. Fig. l3flshows'amethodpf connecting these tubes together. 4

The collecting electrode A is connected to the source of current by means of a resi'stance lt. A can" bedirectlyconnectedto the grid gofthe powr'tube. The cathode C of'tl iis tubepo- Iari'sedby means of alternating current ha in'g' asu-itabl phase withrespec'tto the pha eseiii; ploy'el foi' the' first tube; -I 'l'IYtHei'Q 'apOIariSitIg battery B can be provided. The phase and the tension of polarisation will, for example, be chosen so that the grid has a small negative potential compared with C during the period when the electrode A is receiving electrons.

The two tubes may also be connected by means of. a transformer, a condenser etc., in short all the: methods employed at the present time for coupling vacuum tubes together maybe employed in the present'case.

Another problem consists in supplying the necessary A. C. biasing power to the output electrode. In the case in which biasing was efiected throughout with alternating current, the very high-power necessary for biasing the collecting electrode would be difiicult to obtain, since the biasing power in question must be supplied as a current of very high frequency. It is desirable to use" direct current for the last electrode. As moreover there must be no secondary emission from this electrode, it may be composed of the usual substances and very high tensions may be employed.-

These high tensions cannot be applied without special precautions, because a portion of the electrons might then be attracted directly towards this electrode without passing through the intermediate electrodes. to arrange the electrodes so that this cannot occur. One can thus make use of grids or supplementary electrodes acting as screens. Fig; 14. gives an example of the connections which can be employedin the-latter case.

The relative phases of the alternating currents and the voltages of the batteries which polarise these grids are chosen so that the electrons coming from 4 pass freely, but the electric field produced by C' cannot reach the electrodes [,2 and 3. The field on the electrode 4 will be sufficiently weak to avoid a primary emission.

As the voltage of the source B maybe very high, a rather high voltage amplification can be obtained so as to necessitate only a rather low power in alternating current.

.A third problemis that of obtaining thealternating current for polarising the electrodes, particularly for the last.

Itm'ight. be produced by a common oscillator, the necessary phase angles being obtained-by means of combinations of resistances, condensers andi transformers. In the case of power multiplier tubes a rather high energy would be required; A- small auxiliary multiplier tube can also be employed which in turn receives alternating current from a crystal controlled tube oscillator.

Moreover, one might also employ akind of auto-energisation. A portion of the current emitted by the multiplier is passed. through a filter which isolates the carrier current, but this solution can only be considered in the case in which the lowest modulating frequency is relatively high compared to the carrier or A. C. biasingfrequency, otherwise the problem of filterin'g-would be very'difiicult. This method more orless resemble'sa feedback circuit, but asv the number of secondary electrons does not vary very muchwith the potential of the modulatin'gelectrodes, thereis no critical adjustment. One could further, if necessary; stabilise the potential obtained after filtering.

For very high power it is'necessary to employ very large collecting electrod'esfcooledfor-exampleby water circulation. Fig:-l5rshows a method.

of arrang'ingtheelectrodes;

It is thus necessary controlledby the grid Igr, arrive successively on the electrodes. I, 2, 3, 4, A of increasing surfaces; a grid Ogr separates the electrode A from the others.

These tubes may be arranged as shown in Figs. 11A and 113 with constant tensions on the multiplication electrodes and variable tensions on the intermediate grids.

In the case of a radio amplifier, one can often choose the dimensions and tensions in such a way that the alternating current feeding the multiplier tubes has the frequency. which it is desired to employ for the carrier of the radio emission. A transmitter for the radio-broadcasting of television can thus be composed of a first multiplier tube with photo-electric cell supplying several watts modulated at a frequency corresponding for example to a wave-length of 7 metres. This first tube is connected to a power vmultiplier tube directly connected to the antenna.

In other cases it may be necessary to multiply or demultiply the frequency. In the case in which carrier is not desired, the signals can first of all be detected.

It is understood that the multiplier tubes can serve as amplifiers, oscillators and modulators. The amplifiers may be connected so as to obtain a reaction of the output on the input. If the reaction is negative the stability of the whole can still be increased. These reaction circuits may also be connected to grids or to intermediate electrodes. The whole art of negative reaction amplifiers may be applied here.

What is claimed is:

1. An electron multiplier comprising a source of primary electrons, a plurality of secondary electron emissive electrodes spaced a predetermined distance from one another, and means'for establishing progressively phased variable electric fields between successive secondary emissive electrodes. e

2. An electron multiplier comprising a source of primary electrons, a plurality of secondary electron emissive electrodes'spaced a predetermined distance from one another, and means for producing progressively phased variable potentials and applying said potentials to said secondary emissive electrodes.

3. An electron multiplier comprising a source of primary electrons, a plurality of secondary electron emissive electrodes spaced a predetermined distance from one another along a substantially straight line and arranged in the path of said primary electrons, a collector electrode, sources of variable potentials connected to said electrodes, the phase of the potential applied to each electrode being progressively delayed as viewed from the source of primary electrons, and an output circuit connected to said collector electrode.

4. An electron multiplier comprising a source of primary electrons, a plurality of secondary electron emissive electrodes spaced a predetermined distance from one another along a substantially straight line and arranged in the path of said primary electrons, sources of Variable potentials of the same frequency connected to said electrodes, the phase of the potentialapplied to each electrode being progressively delayed as viewed from the source of primary electrons, and the phase, magnitude, and frequency of said potentials and the spacing of said secondary electron emissive electrodes being so related that a I The electrons emitted by the cathode C and' plurality of groups of electrons are in transit;

through said multiplier at the same time.

5. An electron multiplier comprising a source of primary electrons, a plurality of secondary electron emissive electrodes spaced a predetermined distance from one another along a substantially straight line and arranged in the path of said primary electrons, and sources of sinusoie dally varying potentials of the same frequency connected to said electrons, the phase of the mary electrons.

6. An electron multiplier according to claim 2 further comprising sources of direct current connected to said electrodes.

7. An electron multiplier according to claim. 2 wherein said means-comprise sources of alternating current potentials connected to said electrodes, the phase of said potentials applied to each successive electrode as viewed from the source of primary electrons being progressively delayed 90.

8. An electron multiplier according to claim 2 electrode of the first is receiving mm;

10. In an electron multiplier according to claim 2, further comprising a last electrode, fixedpotential means connected to the last electrode, a screening grid electrode inserted between said last electrode and the next preceding electrode and means for applying a variable potential to said grid electrode such that said grid electrode is normally very negative but is only slightly negative during the periods when secondary electrons are being emitted by the preceding electrode.

11. An electron multiplier according to claim' 1 comprising an electron emitting cathode, aplurality of secondary electron emissive electrodes spaced at predetermined distance from one another, a control grid located between said cathode and the first secondary electron emissive. electrode, and means for producing cyclically varying progressively phased potentials and applying said potentials to said secondary electron emissive electrodes. f

12. An electron multiplier accordingto claim- 1,

in which said electrodes comprise grid-like structures which are supported in parallel planes within an evacuated bulb by a plurality of rods of insulating material. k

13. An electron multiplier according to claim 1 comprising a succession of secondary electron emitting electrodes and anauxiliary grid electrode positioned between successive secondary electron emitting electrodes for the purpos bf controlling the secondary electronstraversing the interelectrode spaces. l

14. An electron multiplier according to claim 1, wherein said secondary emissive electrodes are grid-like structures and further comprisingan evacuated vessel surrounding said electrodes and means for mounting said: grid -like structuresjn parallelplanes comprising a plurality of rodsof insulating material attached to said vessel and said structures.

15. An electron multiplier comprising a source of primary electrons, a plurality of secondary electron emissive electrodes spaced a predetermined distance from one another, a plurality of auxiliary grid electrodes, each of said grid electrodes being positioned between successive secondary electron emissive electrodes, and means for establishing and periodically varying the potentials of said grid electrodes.

16. Anelectron multiplier according to claim 15 further comprising fixed potential means connected to said secondary emissive electrodes.

17. An electron multiplier according to claim 15, wherein the potentials on said grid electrodes have a wave-form and phase such that after a group of electrons in transit from a first secondary electron emissive electrode to a successive electron emissive electrode has passed through said grid electrodes, said grid electrodes have negative potentials with respect to said successive secondary emissive electrodes.

18. An electron multiplier according to claim 15, further comprising a second plurality of grid electrodes, each of said second grid electrodes being positioned between one of said auxiliary grid electrodes and its adjacent successive secondary electron emissive electrode, and fixed potential means connected to said second plurality of grid electrodes.

19. An electron multiplier according to claim 15, in which the distance between an auxiliary grid electrode and the next secondary electron emitting electrode is so chosen that the time taken by an electron in traversing this space is of the order of a half-period of the potential variation on said grid electrode.

20. An electron multiplier comprising a succession of secondary electron emitting electrodes each having a progressively increasing surface area.

21. In a signal modulated carrier wave signalling system an amplifier comprising a source of primary electrons an electron multiplier comprising a plurality of secondary electron emissive electrodes spaced a predetermined distance from one another, and means for producing and applying progressively phased alternating potentials to said electrodes, the frequency of said potentials bearing a predetermined multiple relation to the frequency of the signal-modulated carrier wave to be amplified.

STANISLAS VAN MIERLO. 

